Carbyne complexes applications

The importance of transition metal carbene complexes (compounds with formal M=C bonds) and of transition metal carbyne complexes (compounds with formal M=C bonds) is now well appreciated. Carbene complexes are involved in olefin metathesis (7) and have many applications in organic synthesis (2), while carbyne complexes have similar relevance to... 

Transition metal carbyne complexes are still relatively uncommon as only a few synthetic approaches to these compounds has proved generally applicable. In addition to making the initial characterization (723), the Fischer group has made the largest contribution to carbyne complex chemistry, with some 200 mononuclear complexes of Group 6 and 7 metals having been prepared. 

Thus, the way appears to be open to make carbyne complexes for use in the syntheses of organic compounds. Since to our knowledge there is no specific and selective carbyne source available for preparative purposes, presumably there arises here a wide field of interesting possibilities of application, especially because of the mild conditions required to transfer the carbyne ligand. 

Following the Fischer procedure, alkynyl carbyne complexes trans-X(CO)4M=C—CPh 189 have been obtained in 30-60% yields by reaction of (l-alkynyl)carbene complexes la,b (M = Cr, W) with BX3 (X = Cl, Br, I). To date, (l-alkynyl)carbyne compounds have found application as catalysts as well as stochiometric reagents in organic synthesis.205c-206 Among the transformations reported thus far is the formation of a 4-amino-l-metalla-l-yne-3-ene (= enamino carbyne complex) 190 by addition of HNMe2 to compound 189 (Scheme 79).207... 

The chemistry of metal-carbon triple bonds has developed considerably during the late 1980s. The synthetic basis was broadened, the utility of high-valent metal alkylidynes in metathesis reactions was further developed and refined, and the potential of low-valent carbyne complexes for applications in organic synthesis has become more apparent. The discovery of novel iridium alkylidyne complexes indicates that the full range of metal-carbon triple bonds is not yet known. We can therefore expect that future work in this area of organometallic chemistry will lead to new discoveries with fundamental implications and practical applications. 

Transition metal carbyne complexes are described by the general formula L M=CR where the carbyne ligand (=CR) is bonded to the metal by a metal-carbon triple bond. Transition metal carbene complexes have found numerous applications in synthetic organic chemistry through a variety of carbene transfer and cycloaddition reactions [17]. In contrast, carbyne (L M=CR) and vinylidene (L M=C=CRR ) complexes have far fewer applications, in part because their overall chemistry is significantly less developed [18]. Addition reactions to transition metal vinylidene complexes will be discussed in Chapter 21. The first successful synthesis of a carbyne complex was reported by Fischer and co-workers in 1973 [Eq. (8) 19]. Subsequently, many other carbyne complexes have been synthesized by the classic route of Fischer or by new synthetic methods [20]. 

The first synthesis of a complex containing a metal-carbon triple bond was reported in 1973 [1]. Since then, numerous carbyne complexes have been prepared. In recent years, the study of the reactivity of these complexes has attracted considerable interest [2]. E.g. carbyne complexes have extensively been used as building blocks in the synthesis of transition metal clusters [3]. The coupling of carbyne ligands with CO or isocyanide ligands has also been studied in detail [4]. However, the number of reports on the use of carbyne complexes in synthetic organic chemistry is rather limit in contrast to carbene complexes which have found many applications in the synthesis of carbo- and heterocycles [5]. 

Stable paramagnetic carbynes have also been obtained, usually by electrochemical methods which proved to be successfully applicable to the investigation of the electronic properties of the carbyne ligands (which are shown to behave as rather strong net electron acceptors) and to their activation, in particular towards N-H or C-H bond cleavage. Although electrochemical studies of carbyne complexes have been reported only very rarely, this study illustrates some of their potentialities in this field. 

The possibility of application of stopped flow spectrophotometry to the study of the mechanisms of the formation of the carbyne complexes, involving not a too fast protic attack, has also been demonstrated and indicates that the apparent regiospecific 3-protonation at a vinylidene can in fact occur via different and less straightforward pathways, in particular involving addition to the metal. 

In the 20 years since Principles first appeared the field of organometallic chemistry has continued to expand very rapidly. During the 1950s and 1960s the main theme was the preparation and structural characterization of new compounds, especially those of the transition elements. The last two decades have seen the development of this theme and many unexpected materials such as carbyne complexes and cluster compounds have been discovered. There is now increasing emphasis on the application of organometallics of all types in organic synthesis, both in the laboratory and on an industrial scale. 

Complexes of nucleophilic carbenes are expected to react, like ylids, with electrophiles whereas complexes of electrophilic carbenes are expected to react, like carbocations, with nucleophiles and bases. All the complexes of terminal carbenes have in common the reactions with olefins, although their nature also varies. The principles of these reactions are detailed here, and application in catalysis and organic synthesis, are exposed in Parts IV and V respectively. Reactions of metal-carbene complexes leading to metal-carbyne complexes are mentioned in section 2. 

The application of the nucleophilic or electrophilic induced carbonyl carbyne coupling reaction has been perfectly demonstrated by E. O. Fischer [13,14], R. R. Schrock [15], J. L. Templeton [16,17], F. G. A. Stone [18], Angelici [19,20], A. Mayr [21,22], R. J. G. L. Geoffroy [23] and S. J. Lippard [24], preparing a variety of interesting ketenyl, alkyne and binuclear complexes involving r 2-ketenyl species as possible intermediates. 

Many other species are stabilized in 18-electron organometallic complexes car-benes and carbynes, enyls and polyenyls (XL ligands), o-xylylene (o-quinodime-thane), trimethylenemethane, benzyne, norbornadiene-7-one, cyclohexyne, 1,2-di-hydropyridines (intermediates in biological processes), thermodynamically unfavorable organic tautomers such as vinyl alcohols [less stable by 14 kcafrmol (58.5 kJ mol ) than their aldehyde tautomers], aromatic anions resulting from deprotonation in juxta-cyclic position such as tautomers of phenolates and benzylic carbanions. All these species have a specific reactivity that can lead to synthetic applications in the same way as cyclobutadiene above. 

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